How Much Concrete Do I Need? Formula, Bags and Yards (2026)

How much concrete do I need? Multiply length in feet by width in feet by thickness in inches divided by 12 to get cubic feet, then divide by 27 to get cubic yards. A 10x10 slab poured 4 inches thick works out to 33.3 cubic feet, or 1.23 cubic yards, which is 56 of the 80 pound bags.

That one formula covers almost every pour a homeowner will ever do. Slabs, footings, post holes, steps, pads. The shapes change, but the math does not. Below is the whole thing broken down, plus the bag counts and the judgment calls that trip people up on pour day.

How do you calculate concrete?

Concrete is sold by volume, not by area. So every calculation is the same three steps: measure your space, convert everything into one unit, then divide down to yards.

• Step 1: cubic feet = length (ft) x width (ft) x (thickness in inches / 12)
• Step 2: cubic yards = cubic feet / 27
• Step 3: add 5 to 10 percent for waste

The divide-by-12 step is where people go wrong. Thickness is almost always given in inches while length and width are in feet, so you have to convert. A 4 inch slab is 0.333 feet thick. A 6 inch slab is 0.5 feet thick.

Run a 12x12 pad at 4 inches through it: 12 x 12 x 0.333 = 48.0 cubic feet. Divide by 27 and you get 1.78 cubic yards, which is 80 of the 80 pound bags. If you would rather skip the arithmetic, punch the numbers into the Concrete Calculator and it will hand you cubic feet, cubic yards and bag counts at once.

How much concrete do I need for a slab?

Slabs are the most common pour, and thickness drives the whole number. Going from 4 inches to 6 inches on the same footprint adds 50 percent more concrete. That is not a rounding error. That is the difference between a trunk full of bags and a truck.

Here is the same 10x10 footprint at three thicknesses:

• 10x10 at 4 inches: 33.3 cu ft = 1.23 cu yd = 56 x 80lb bags
• 10x10 at 5 inches: 41.7 cu ft = 1.54 cu yd = 70 x 80lb bags
• 10x10 at 6 inches: 50.0 cu ft = 1.85 cu yd = 84 x 80lb bags

And a bigger footprint, the 12x12:

• 12x12 at 4 inches: 48.0 cu ft = 1.78 cu yd = 80 x 80lb bags
• 12x12 at 6 inches: 72.0 cu ft = 2.67 cu yd = 120 x 80lb bags

Four inches is the standard for patios, walkways and shed pads. Six inches is what you want under vehicles, heavy equipment or a driveway apron. If you are stuck between the two, read how thick a concrete slab should be before you order, because you cannot add thickness later. For a full breakdown of that first example, we walk through the bag count for a 10x10 slab step by step.

One thing worth saying out loud: your subgrade is never perfectly flat. A slab you measured at 4 inches will average closer to 4.25 or 4.5 in real life. That is the main reason waste allowance exists.

How many bags of concrete are in a cubic yard?

A cubic yard is 27 cubic feet. Each bag size yields a set volume of mixed concrete, so the bag count per yard is fixed:

• 80 lb bag: 0.60 cu ft each, so 45 bags per cubic yard
• 60 lb bag: 0.45 cu ft each, so 60 bags per cubic yard
• 50 lb bag: 0.375 cu ft each
• 40 lb bag: 0.30 cu ft each, so 90 bags per cubic yard

Notice the yield is a lot smaller than people expect. An 80 pound bag makes about six tenths of a cubic foot. That is a bucket, not a wheelbarrow. Forty five of them make one yard, and one yard is a small pour. We go deeper on yields in how many cubic feet are in a bag of concrete.

To convert your own project: take your cubic feet and divide by the bag yield. Sixty cubic feet divided by 0.60 is 100 bags of 80 pound mix. Or let the Concrete Calculator switch bag sizes for you and compare all four at once.

How much concrete do I need per fence post?

Fence posts eat more concrete than anyone plans for. A typical hole is 10 inches in diameter and 24 inches deep. Drop a 4x4 post in the middle and the concrete that actually fills the hole is 0.92 cubic feet net. That rounds to 2 of the 80 pound bags per post.

Do the multiplication before you shop. Twenty posts is 40 bags. That is nearly a full yard of material for a fence you were thinking of as a weekend job.

• 1 post: 2 x 80lb bags
• 10 posts: 20 x 80lb bags
• 20 posts: 40 x 80lb bags

Wider or deeper holes climb fast, because volume scales with the square of the diameter. Gate posts and corner posts usually get both. See bags of concrete per fence post for hole sizes beyond the standard, then check your total with the Concrete Calculator.

How much concrete do I need for footings and piers?

Footings are just cylinders or rectangles, so the same formula applies with a round-shape tweak. A standard round footing 12 inches in diameter and 48 inches deep holds 3.14 cubic feet, which is 6 of the 80 pound bags.

For round holes, use radius x radius x 3.14 x depth, with every measurement in feet. A 12 inch diameter is a 6 inch radius, or 0.5 feet. Depth of 48 inches is 4 feet. That gives 0.5 x 0.5 x 3.14 x 4 = 3.14 cubic feet. Clean.

Deck builds are where this stacks up. Nine piers at 6 bags each is 54 bags before you have poured a single board. Strip footings under a wall use the plain rectangular formula: length x width x depth, all in feet. More shapes and sizes are covered in how much concrete you need for footings.

Should you order ready-mix or mix bags?

The dividing line is roughly one cubic yard. Under a yard, bags usually win. Over a yard, a truck usually wins, and your back definitely wins.

A ready-mix truck carries a full load of about 8 to 10 cubic yards. Suppliers will deliver less than that, but short-load fees typically apply under about 1 cubic yard, and those fees can wipe out the savings on a small pour. Call your local plant and ask where their threshold sits before you decide.

• Choose bags when: the pour is under about 1 cubic yard, access is tight, or you want to work in stages
• Choose ready-mix when: the pour is over about 1 cubic yard, needs to be monolithic, or a truck can reach the forms
• The hidden cost of bags: mixing time, a mixer rental, and hauling 45 bags per yard

There is a quality argument too. A slab poured from one truck cures as a single unit. A slab poured from 84 bags mixed in batches has cold joints if you are slow. For a big flat pour, that matters. We compare both routes in detail in concrete bags vs ready-mix.

We are not quoting prices here on purpose. Concrete pricing swings by region, by season and by the week. Get a current quote from your supplier and compare it against bag pricing at your local yard on the same day.

How do you measure odd shapes?

Break the shape into rectangles. That is the whole trick. An L-shaped patio is two rectangles. A patio with a bump-out is a rectangle plus a rectangle. Calculate each piece separately, add the cubic feet, then divide by 27 once at the end.

For circles, use radius x radius x 3.14 x depth in feet. For a triangle, take half of base x height, then multiply by depth. For a curved edge, square it off on paper and accept that you will have a little extra. Extra is fine. Running short is not.

If you are pinning down the footprint first, the Square Footage Calculator will handle the area piece, and then you only need to apply thickness and divide by 27.

Measuring tips that save a pour

• Measure the forms, not the drawing. Forms move.
• Check depth at several points, not just one corner.
• Write down every measurement in feet before you start multiplying.
• Measure twice, on two different days if the ground has been rained on.

How much extra concrete should you order?

Add 5 to 10 percent for waste, spillage and uneven subgrade. Every time. This is not padding, it is planning.

Concrete disappears into places you did not account for. Low spots in the base. Material stuck in the mixer or the wheelbarrow. A form that bows a half inch under load. Spills. On a 1.23 cubic yard slab, 10 percent is only about a tenth of a yard, which is a handful of bags. Cheap insurance.

Lean toward 10 percent when the subgrade is rough, hand-dug, or you are pouring into holes. Lean toward 5 percent when you have a compacted gravel base and clean forms. And remember: running out mid-pour means a cold joint in your slab, and there is no fixing that after it sets.

Quick reference

• 1 cubic yard = 27 cu ft = 45 x 80lb bags = 60 x 60lb bags = 90 x 40lb bags
• 10x10 slab, 4 inches = 1.23 cu yd = 56 x 80lb bags
• 12x12 slab, 4 inches = 1.78 cu yd = 80 x 80lb bags
• 12x12 slab, 6 inches = 2.67 cu yd = 120 x 80lb bags
• Fence post, 10in x 24in hole = 0.92 cu ft net = 2 x 80lb bags
• Round footing, 12in x 48in = 3.14 cu ft = 6 x 80lb bags
• Ready-mix truck = about 8 to 10 cu yd full load

Bottom line

How much concrete do you need? Cubic feet = length (ft) x width (ft) x (thickness in inches / 12). Cubic yards = cubic feet / 27. Then add 5 to 10 percent for waste. A 10x10 slab at 4 inches is 1.23 cubic yards, or 56 of the 80 pound bags. Under about a yard, buy bags. Over a yard, call for a truck and skip the mixing.

Measure your forms, run the numbers through the Concrete Calculator, and confirm current pricing with your local supplier before you order.